Method of automatically cooking food with forced hot air

ABSTRACT

An automatic control system for regulating the cooking process of a food product where it reaches the desired result respective of the initial temperature of the food product, its moisture content or quality. The invention solves the problem by using the characteristic breakpoints (2,3,4,5) that, whereby the adding of heat, occurs on one or two temperature curves (12,13) obtained at different measuring points, when the temperature again rises after the temperature drop of the heat transportation of medium after the insertion of the food product with a lower temperature than the heat transportation medium or when water transites from solid to liquid form (2,3) and from liquid form to steam (4,5).

It is a know fact that one by preparing food in an oven for eachindividual food product must set the temperature and time controls.These parameters vary according to the type of product placed in theoven and its volume. Additionaly the temperature of the foodproduct whenstarting the process affects the amount of time necessary for theprocess. The setting of these two basic parameters is generally a matterof subjective judgement and/or based on experience. The oven temperatureis mostly controlled by a temperature controlling device with thepurpose of keeping the set cooking temperature as constant as possible.

Apart from these basic settings there is often a need for, whenprocessing certain types of foodproducts, removing water in the form ofwater vapour, and again in other cases adding water vapour. The settingsof these parameters are mainly done manually by a through the entirecooking process engaged function of removing or adding water vapour. Thesettings can also be in advance fixed controlling programs.

These drawbacks are particularly troublesome when repetitly processingvarying volume of the same type of foodproduct. This is the case oftenfound in catering environments.

As one example among many others the course of events when processingfrench fries can be described as follows;

In a rotating hot air oven a continuously varying volume of french friesis processed in hot air. Initially the french fries being cooked givesoff large amounts of water vapour that helpes thawing by allowingincreased heat transport, but at a later stage the vapour can prevent acrisp surface being reached. The time of processing varies in accordanceto the volume of french fries being processed and the amount of watervapour given off. The amount of water vapour given off is dependent onhow the frozen or refrigerator chilled french fries is manufactured,frozen or stored. It is obvious that it is virtually impossible tomanualy set all the required parameters needed for reaching a nicelytasting product.

Having to do this by subjective judgments in a stressful workenvironment very often results in mistakes giving that given qualitydemands is not reached or even that the food product is destroyed.

The present invention eliminates demand for continous new settings andthereby the risc of failure.

The invention is based on the fact that a food product to be cooked inheat always starts off at a lower temperature than the temperature ofthe heat transportation medium into which the food product is placed.This results in caracteristic changes in shape of the temperature curveof the heat transportation medium when the food product is placed intoit.

On the enclosed drawings,

FIGS. 1 to 3, curves are shown illustrating temperature changes overtime.

FIG. 4 shows a general outline of an oven for processing chilled orfrozen food e. g. french fries.

The caracteristic changes in shape of the temperature curve is seen asbreaking points which depend on differences in the initial temperatureof the food product, the water content in the form of water or ice, thespecific heat of the inserted food products or a combination of thesedifferent properties. When a frozen product is placed in the oven, twovery characteristic breakpoints, 2 or 3 together with 4 or 5, areobtained. Where the first one, 2 or 3 appears the ice transites intowater (melting point) and where the other, 4 or 5, appears, the watertransites into steam (boiling heat). These break points can therefore beused for controlling amongst other things a device which regulates theamount of water vapour in the heat transportation medium circulating inthe oven compartment. Most food products are in the initial stages ofthe cooking process, especially when frozen but even cold, almostunaffected by a heat transportation medium saturated with water vapour.A heat transportation medium saturated with watervapour is able to carrylarger amounts of heat compared to a dry heat transportation medium.These conditions can be used to rapidly raise the temperature of thefrench fries. When inserting a chilled food product a quick andsubstantial temperature drop in the vapour saturated heat transportationmedium circulating in the ovencompartment occurs. This drop appearsdespite the by the heating device continuosly added heat. This isindependent of whether the food product is frozen, in which case thebreakpoint of the temperature curve is found in the lower parts of thetemperature scale, or the food product is room temperature, in whichcase the break point of the temperature curve is found in the upper partof the temperature scale. The ventilator 7 connected to the ovencompartment 6 is shut during the heating phase and thereby the heattransportation medium is saturated with water from either the foodproduct or from the outside in the form of water or steam. Thereby theheat transportation medium manages to transport large amounts of heat,and it does not absorb more water and thereby not drying the foodproduct.

The temperature can be measured at several points along the path 19 ofthe heat conducting medium, that is forced around by the fan 8 or anequivalent device. These measuring points produce different temperaturecurves according to where in the heat transportation medium the readingsare taken. The biggest disparity is seen between the two curves whostemperatures are measured after the passage through the heating devices(oven temperature that is), measuring point 9, and after the passagepast or through the food product before the heating elements (intacetemperature that is), measuring point 10. It is a caracteristic that forany given food product, the shape of the temperature curves will beconstant and that these curves always are situated in a fixed positioninbetween. The curvforms show, dependent on the inserted volume of foodproduct a significant expansion or compression along the time ortemperature axis without changing the information in the curves. CompareFIGS. 1 and 2 that applies for a larger volume of food product with FIG.3 that applies for a smaller volume of the same food product. When theoven temperature, that is the temperature of the circulating heattransportation medium after passage through the heating device 11, againreaches the desired value, equal to the maximum temperature allowed fora certain food product, it is for a vide range of food productsnecessary to reduce the water vapour of the heat transportation mediumso as to avoid the food product absorbing water, render it soft andsoggy. The after this phase added heat energy is mainly used forchanging the surface of the food product e. g. brown it.

Having reached the pre-set temperature, the ventilator 7 is opened,expelling the water vapour saturated heat transportation medium duringthe period 15 from the breakpoint 4, and drawing in dry heattransportation medium for facilitating surface changes e. g. browning.

It has in many cases become apparent, that the preparation time can bereduced, together with a substantial raise in food product quality, ifthe heat treatment with hot air saturated with water vapour can continuefor a little while longer after you have reached desired temperature,before letting the redudant water vapour out. The prolonging results ina higher temperature deeper in the food product which thereby have timeto get thoroughly cooked. This can be done by a certain delay after thebreakpoint 4, before opening the ventilator 7, or the ventilaton time 16can start off from the breakpoint 5, or at a certain delay from this.

The shape of the curves, i. e. curve 12 showing the oven temperature andcurve 13 showing the intake temperature, along the time axis is variableaccording to the volume of the food product being cooked as well as itstemperature when placing it in the oven compartment. This means that ifonly one of the break points 4 or 5 is used for a proportionate controlof the opening or closing of the ventilator 7 you get a control thatdoes not allow correct expelling time for different volumes andtemperatures of the inserted food product. However using the relationsbetween the breakpoints 4 and 5 along the time axis and/or temperaturaxis automatic control is attained, because information of both volumeand temperature is obtained.

More accurate control can be achieved if data of the breakpoints 2 or 3is used together with data of the breakpoints 4 or 5.

By measuring the rate of convergence of the curves 12 and 13 towards theconvergence point 17 and comparing this with the relative positons ofthe breakpoints 2 or 3, and/or the breakpoints 4 or 5, it is possibleto, independent of the volume and initial temperature, determine thetotal cooking time. This makes manually set cooking times according tosome predetermined point on any of the temperature curves obsolete, acondition that used to be general practice.

The basic regulation of the automatic control system as described, isset irrespective of whether one or both of the temperature curves 12 or13 is used. This basic control can be achieved by means of relays ormicroprocessors. The microprocessors offers greater flexibility becauseapart from its fixed memory (ROM) for its basic functions, additionalchanges can be made via a keyboard in the microprocessors flexiblememory (RAM). A variety of changes can be made and stored in thecomputers memory. Thus the automatic control system can be programmedfor a list of different, regulary used products. Via a control panel theoperator can choose the programme for a given product and the processruns automatically independent of the initial temperature, volume orwater content of the product to be cooked.

Position Numbers and Denomination of Details

The parenthesis behind the denomination describes on which figures theposition numbers can be found.

1 Starting point (1, 2, 3)

2 Temperature curve breakpoint of oven temperature after temperaturedrop (1, 2, 3)

3 Temperature curve breakpoint measured after passage through or by thefood product (1, 2, 3)

4 Temperature curve breakpoint of oven temperature when balance hasreoccured (1, 2, 3)

5 Temperature curve breakpoint measured after passage throuh or by thefood product (intake temperature) (1, 2, 3)

6 Oven compartment (4)

7 Ventilation device (4)

8 Fan (4)

9 Measuring point for oven temperature (4)

10 Temperature curve measuring point measured after passage through orby the food product (4)

11 Heating element (4)

12 Actual temperature curve (oven temperature) (1, 2, 3)

13 Temperature curve measured after passage through or by the foodproduct (intake temperature) (1, 2, 3)

14 Food product (4)

15 Expelling time when basing control on temperature curve 12 (1, 2, 3)

16 Expelling time when basing control on temperature curve 13 (1, 2, 3)

17 Convergence point of the both temperature curves (1, 2, 3)

18 Airing time when basing control on the relation between the differentbreakpoints (3)

19 Course of the heat transportation medium (4)

I claim:
 1. An automated method for controlling, in a forced air ovenwhich uses heated air as a heat transport medium, a process for cookingand heating a food product placed within said oven, said processautomatically adjusted according to characteristic parameters of thefood product, said oven comprising an oven compartment, a fan forforcibly blowing air through said compartment, at least one heatingelement for supplying heat to the air, a ventilator connected to saidoven compartment for communicating said air out of said oven compartmentwhen said ventilator is in an open position and for retaining said airwhen in a closed position, a plurality of temperature detecting devicespositioned within said oven compartment along a flow path of said hotair, said devices for measuring at least the air temperature of airdisplaced from said fan and after passage over and through said foodproduct, and a control unit operably connected to each of saidventilator, temperature detecting devices and heating elements, saidcontrol unit including a multitude of pre-programmed temperature curvesfor different kinds of food products, wherein each of said respectivetemperature curves includes at least two phase-change points, one ofsaid phase-change points defined as a sensed temperature where ice isconverted to water and the second phase-change point defined as a sensedtemperature where water is converted into steam, comprising the stepsof:closing said ventilator for retaining residual humidity within saidoven compartment; activating the fan and heating elements to force saidsaturated air to circulate about said oven compartment, whereby saidheating elements heat said air to a desired temperature before said airis passed over said food product, said desired temperature a maximumtemperature for said food product; continuously measuring thetemperature of the air at the points in the flow path immediately afterthe heating elements and immediately after said air has passed over thefood product; sending the measured temperatures in signal form to saidcontrol unit for comparing said measured temperatures to saidtemperatures two phase-change points on said pre-programmed temperaturecurve for the kind of food product being cooked; maintaining theactivation of the heating element if the measured temperature is notequal to at least one of the phase-change point temperatures; openingsaid ventilator when said measured air temperature of the air which haspassed over said food product is equal to said predetermined secondphase-change point temperature, whereby said control unit ends saidcooking process by deactivating said fan and heating elements.
 2. Themethod for preparing food as claimed in claim 1, wherein the time ofcooking is controlled by the water content in the heat transport mediumby means of the cooking process being regulated by use of phase-changecharacteristics for water in a temperature curve which is measured afterthe heat transport medium has passed by the food product and when waterchanges phase from solid ice to liquid water and when said same wateragain chances phase from liquid to steam.
 3. The method for preparingfood as claimed in claim 1, wherein the process is regulated using phasechange points in a first temperature curve which is measured after theheat transport medium has passed by the food product, and wherein thepoints on said first wave are compared to the phase-change points in atemperature curve which is measured before the heat transport mediumpasses by the food product when water changes phase from solid to liquidform and when water changes phase from liquid to steam.
 4. The methodfor preparing food as claimed in claim 1, wherein control of the watercontent in the heat transport medium is regulated by comparing the phasechange temperature points in the pre-programmed temperature curve to atemperature of the heat transport medium once it has passed the foodproduct.
 5. The method for preparing food as claimed in claim 1, whereinthe temperature measurement of the heat transport medium is performed attwo different points in the flow path, one point immediately after theheat transport medium passes the heating elements and the other afterthe heat transport medium passes the food product, whereby temperaturecurves are obtained, and used to regulate heat supply, ventilation andtermination of food processing.
 6. The method for preparing food asclaimed in claim 1, wherein a convergence point between thepre-programmed temperature curve and the measured temperature curve ofboth curves is used for controlling the termination of food processing.7. The method for preparing food as claimed in claim 1, wherein acontrol of the cooking process is regulated by a relationship betweenthe pre-programmed and actually measured temperature curves obtained bycomparative measurements performed along one of the time and temperaturecurves and the curves in combination.
 8. The method for preparing foodas claimed in claim 1, wherein a control of the cooking process isregulated using only the relationship between the pre-programmed andactually measured temperature curves by comparative measurementsperformed along one of the time and temperature axis and the curves incombination.
 9. The method for preparing food as claimed in claim 1,wherein at least two phase-change points used at a time and incombination form a basis for regulating other functions than ventilationand duration.
 10. The method for preparing food as claimed in claim 1,wherein use of the phase-change points occurs on the time andtemperature axis of the temperature curves together as a basis for atime and temperature operating change.
 11. The method for preparing foodas claimed in claim 1, further including the step of maintainingactivation of said heating elements until said air temperature withinsaid oven compartment reaches said desired temperature in order to brownsaid food product.